Glass-ceramics are well known, synthetic, inorganic materials. In general, they are produced by melting an appropriate precursor glass and developing at least one crystal phase, in situ, in the glass. This is usually accomplished by a thermal treatment that converts a portion of the glass to one or more crystal phases that are uniformly dispersed in a matrix glass. Glass-ceramics are commonly designated by the name or nature of their predominant, or primary, crystal phase.
Numerous different types of glass-ceramics, that provide widely different properties, have been developed. Thus, glass-ceramics having coefficients of thermal expansion (CTEs) that vary from very high positive coefficients to relatively low negative coefficients are described in the prior art. While not so limited, the present invention is particularly concerned with aluminosilicate glass-ceramics, in particular, a beta-eucryptite glass-ceramic having a negative CTE.
Beta-eucryptite, glass-ceramic bodies have found applications in telecommunication components. One such application is in Bragg gratings in fibers that are a thermal, that is, are temperature compensating. For example, as shown in FIG. 1, one such device takes the form of a fiber having a positive CTE that is mounted on a substrate having a negative CTE. This arrangement compensates for a refractive index change in the fiber as well as its positive CTE. Bragg gratings in fibers and their production are described in detail in U.S. Pat. No. 5,104,209 (Hill et al.) and U.S. Pat. No. 5,351,321 (Snitzer et al.). For this use, it has been found desirable to employ an aluminosilicate glass-ceramic, in particular, a beta-eucryptite glass-ceramic having a negative CTE.
Unfortunately, aluminosilicate glass-ceramics tend to exhibit highly microcracked, porous structures that provide high surface areas. When this feature is combined with the highly polar nature of the glass-ceramic composition, the result is a strong tendency to absorb moisture in the microcracked structure. The reaction of the moisture with a polar component in the glass-ceramic, in particular, alumina, causes the glass-ceramic to grow dimensionally. Such growth, of course, renders the material unsatisfactory for use as a grating.
The reaction of the glass-ceramic with moisture also tends to alter the composition of the glass-ceramic and, consequently, its CTE value. This, of course, is a further, destabilizing effect that is undesired.
It is, therefore, a basic purpose of the present invention to provide a method of stabilizing a microcracked glass-ceramic, in particular, an aluminosilicate glass-ceramic, against a hot, humid atmosphere with minimal change in dimension and CTE value due to reaction with moisture.
Another purpose is to provide a stabilized, glass-ceramic component for use in telecommunication equipment.
A further purpose is to provide a method of treating a microcracked glass-ceramic to enable its use with minimal change in either dimension or CTE due to reaction with moisture.
A specific purpose is to provide a method of treating a microcracked glass-ceramic material having a negative CTE to permit its use in producing a temperature-compensating Bragg grating in a fiber.
Another specific purpose is to provide a temperature-compensating, Bragg grating adapted to use in a hot, humid atmosphere.
The invention resides in part in a method of stabilizing a glass-ceramic body, that has a large, internal surface area, against change in dimension and/or CTE value when the body is exposed to a humid atmosphere, the method comprising coating the body with an aqueous solution of an alkali metasilicate, a non-aqueous solution of a silane that is strongly non-polar with respect to water, or successive coatings of the metasilicate and the silane.
The invention further resides in a glass-ceramic body having a large, exposed, internal surface area that is exposed to external atmospheric conditions, the body being coated with an aqueous solution of sodium metasilicate, a non-aqueous solution of a silane that is strongly non-polar with respect to water, or a combination of the metasilicate and the silane, whereby the body is stabilized against change in dimension and/or CTE value.